1. Field of the Invention
This invention relates to a liquid crystal display device having a display panel used to perform video signal display corresponding to a video signal for each frame period, for example, and perform non-video signal display which does not correspond to the video signal.
2. Description of the Related Art
A flat-panel display device represented by a liquid crystal display device is widely used to display images in a computer, car navigation system, television receiver or the like. Generally, the liquid crystal display device includes a liquid crystal display panel having a matrix array of liquid crystal pixels, a backlight which illuminates the liquid crystal display panel and a display control circuit which controls the liquid crystal display panel and backlight.
The liquid crystal display panel has a structure in which a liquid crystal layer is held between an array substrate and a counter-substrate. Generally, the array substrate has a plurality of pixel electrodes substantially arranged in a matrix form, a plurality of gate lines arranged along the rows of pixel electrodes, a plurality of source lines arranged along the columns of pixel electrodes, and thin film transistors (TFT) arranged as pixel switching elements near the intersections between the gate lines and the source lines. Each thin film transistor is made conductive to apply the potential of a corresponding source line to a corresponding pixel electrode when a corresponding gate line is driven. The counter-substrate has a color filter and a common electrode arranged to cover the color filter and face the pixel electrodes. A pair of the pixel electrode and common electrode is associated with a pixel area which is part of the liquid crystal layer located between the electrodes to configure a liquid crystal pixel. A potential difference between the pixel electrode and the common electrode is held as a liquid crystal drive voltage after the thin film transistor is made nonconductive and controls the liquid crystal molecular orientation in the pixel area by use of an electric field corresponding to the liquid crystal drive voltage. In the above control operation, when the liquid crystal molecular orientation is controlled by use of the one-directional electric field, the liquid crystal molecules are unevenly distributed in the liquid crystal layer and finally set into an uncontrollable state. When the potential of the common electrode is constant, for example, the potential of the pixel electrode is set to periodically invert the polarity of the liquid crystal drive voltage between the common electrode and the pixel electrode for every preset number of horizontal periods (H) in addition to one frame period (V=vertical period) in order to prevent occurrence of the uneven distribution.
The display control circuit includes a gate driver which drives the gate lines, a source driver which drives the source lines by the pixel voltages for the pixel electrodes of the pixels (horizontal pixel line) of a row corresponding to the gate line driven by the gate driver and a controller circuit which controls the operation timings of the gate driver and source driver.
In the field of large-scale liquid crystal television receivers, a liquid crystal display panel of an OCB (Optically Compensated Bend) mode having the high-speed liquid crystal response characteristic required for moving image display is adopted. The liquid crystal display panel performs the display operation in an alignment state of the liquid crystal molecules previously transitioned from the splay alignment to the bend alignment. The bend alignment is reversely transitioned to the splay alignment when a voltage-non-applied state or a state close to the voltage-non-applied state is maintained for a long period of time. In the above liquid crystal display panel, black insertion driving is used with the intention of preventing reverse transition to the splay alignment (refer to Jpn. Pat. Appln. KOKAI Publication No. 2002-202491). In this case, the liquid crystal display panel is driven to perform the video signal display in a period corresponding to approximately 80%, for example, of one frame period and perform the black display (non-video signal display) in which the liquid crystal drive voltage becomes the maximum in the remaining period corresponding to approximately 20% of one frame period. Further, the black insertion driving
The black insertion driving provides discrete pseudo-impulse response of luminance similar to a CRT in a moving image display. This is effective to clear the retinal persistence occurring on viewer's vision and display the movement of an object smoothly.
FIG. 13 shows an example of black insertion driving of a 4H1V inversion type in which the polarity of the liquid crystal drive voltage is inverted in units of four horizontal periods and in units of one frame period. In the black insertion driving, the gate lines Y1, Y2, Y3, Y4, . . . should be scanned twice in total for each frame period to perform black insertion writing and video signal writing. The gate lines Y1, Y2, Y3, Y4, . . . are divided into groups of four lines, sequentially driven at the rate of one group for every 4H for black insertion writing and driven at the rate of one group for every 4H for video signal writing with a delay of the black insertion period (approximately 20% of one frame period) from the start of the black insertion writing. At this time, in order to prevent collision between the black insertion writing and video signal writing, each group is driven during the first one of 4H/5 periods obtained by equally dividing 4H assigned to the group for black insertion writing by five and driven during the second, third, fourth and fifth ones of 4H/5 periods obtained by equally dividing 4H assigned to the group for video signal writing by five. As shown in FIG. 13, the gate driver outputs four gate pulses in parallel to drive the gate lines Y1 to Y4, Y5 to Y8, . . . of each group for black insertion writing and sequentially outputs four gate pulses to drive the gate lines Y1 to Y4, Y5 to Y8, . . . of each group for video signal writing. The source driver converts black signals (non-video signals) for a corresponding horizontal pixel line into pixel voltages and outputs the thus converted pixel voltages to the source lines X1 . . . in parallel when the gate lines Y1 to Y4, Y5 to Y8, . . . of each group are driven for black insertion writing. Further, it converts video signals for a corresponding horizontal pixel line into pixel voltages and outputs the thus converted pixel voltages to all of the source lines X1 . . . in parallel when each of the gate lines Y1 to Y4, Y5 to Y8, . . . is driven for video signal writing. Thus, every 4-row liquid crystal pixels (four horizontal pixel lines) are simultaneously subjected to the black insertion writing performed in the first 4H/5 period which is contained in the four horizontal periods assigned thereto, and subjected to the video signal writing performed in the second, third, fourth and fifth 4H/5 periods which are contained in the four horizontal periods assigned thereto. The pixel voltage polarity is inverted in units of four horizontal pixel lines and in units of all the horizontal pixel lines. Further, it is preferable that the pixel voltage polarity is inverted for each pixel in each horizontal pixel line. In the above black insertion driving, the writing operation is performed five times for every four horizontal periods. Thus, the black insertion driving is referred to as a 1.25×-speed driving operation in contrast to a driving operation in which the video signal writing is performed one time for each horizontal period without performing the black insertion writing.
As another example of the black insertion driving, a 1.5× speed driving operation in which the writing operation is performed three times (one black insertion writing operation and two video signal writing operations) for every two horizontal periods and a double speed driving operation in which the writing operation is performed two times (one black insertion writing operation and one video signal writing operation) for each horizontal period are considered, for example. Generally, when n is a natural number, an (n+1)/n X-speed driving operation in which the writing operation is performed (n+1) times (one black insertion writing operation and n video signal writing operations) for every n horizontal periods is considered. If n is increased, the ratio of the total black insertion writing period to the total video signal writing period can be reduced. However, the increase of n increases a difference between the black insertion periods for the horizontal pixel lines corresponding to the gate lines of each group. If n is set to 4 as shown in the example of the black insertion driving shown in FIG. 13, a difference of three horizontal periods occurs between the black insertion periods for the horizontal pixel lines corresponding to the gate lines Y1 and Y4, for example. According to our experiments, it was confirmed that the quality of a display image on the display panel was not deteriorated due to a difference between the black insertion periods at the time of n=4. On the other hand, the result that a difference between the black insertion periods was recognized as a black stripe due to a difference in the luminance on the display panel was obtained at the time of n≧5. Therefore, it is preferable to set n to 4 or less, that is, n=1, 2, 3 or 4.
When the 4H1V inversion type black insertion driving is applied to the large-scale liquid crystal display panel, for example, the following problem occurs when a video signal for intermediate gradation display is written into all the pixels. In the large-scale liquid crystal display panel, since the time constant of the source line which acts as a load of the source driver, that is, the load capacitance is large, the video signal writing period for one horizontal pixel line is terminated in some cases before potentials of the entire source lines are transitioned to the intermediate gradation display level by the first video signal writing following after the black insertion writing. In other words, the video signal writing period becomes insufficient for the length required for transition of the source line potential. Specifically, the video signal writings for four horizontal pixel lines are sequentially performed after the black insertion writing, but in this case, the luminance of the first horizontal pixel line becomes lower than the luminance of the remaining three horizontal pixel lines and this is recognized as a lateral stripe. The lateral stripe occurs in units of four horizontal pixel lines in the liquid crystal display panel. In general, when the video signal writings for n horizontal pixel lines are sequentially performed after the black insertion writing, the lateral stripe occurs in units of n horizontal pixel lines (refer to Jpn. Pat. Appln. KOKAI Publication No. 2003-280036).
Further, a multiplexer is provided on the liquid crystal display panel in some cases in order to reduce the circuit scale of the source driver. For example, when the number of output terminals of the source driver is reduced to half the number of source lines, the multiplexer connects all of the output terminals of the source driver to half of the source lines in the first half of the video signal writing period for each horizontal pixel line and connects all of the output terminals of the source driver to the remaining half of the source lines in the latter half of the video signal writing period. That is, each horizontal pixel line is driven in two separate cycles. If the black insertion driving is performed in addition to the division driving, the video signal writing period is reduced to half in comparison with a case wherein the division driving is not performed and a pixel voltage writing error due to insufficiency of the video signal writing period becomes significant. Therefore, occurrence of the lateral stripe becomes serious due to utilization of the multiplexer.
Conventionally, when the video signal writing is performed after the non-video signal writing, there occurs a problem that the lateral stripe is generated.